As written, Thousand Suns is a toolbox game. It provides only a very thin setting based on the tropes of what I call "imperial science fiction," which is to say, SF stories written from the 1940s to the 1970s by authors like Poul Anderson, Bertram Chandler, H. Beam Piper, and E.C. Tubb, among many others. The game doesn't even specify whether the primary galactic government is a federation or an empire and in fact provides for either option.
Consequently, in this campaign, I'll be doing something very similar to what I'd done with Dwimmermount: making up the details as I go along, using my players' ideas and the actions of their characters as a springboard for setting development. So, I started with a basic jumpline map of the frontier sector of Limzano and established only that the Federation -- I decided to go with a corrupt but democratic galactic government rather than the more clichéd benevolent but autocratic empire -- laid claim to the whole sector but only exercised direct control over about half its worlds. Sizable portions are in the hands of two non-Federation powers (one of the alien) and another chunk is made up of independent worlds founded by Terran colonists who left the Federation in hopes of making a better life for themselves. Beyond that, I didn't tell my players much else and figured, as I did with Dwimmermount, that it'd be more fun to let the setting evolve according to the logic and demands of actual play.
The first character created was Malos Vermilion. He hails from a "barbarian" world beyond Federation space. This world was colonized by genetically engineered soldiers who eventually reverted to a pre-industrial, animistic society subject to periodic raiding by space pirates and slavers. His player decided that Malos was captured on one of these raids but soon impressed his captors with his courage and skill at arms. They eventually trained him to be part of their crew, learning the ins and outs of starship operation and other high-tech vocations. Ambitious, Malos wanted a ship of his own, which he got through a deal with a sector-wide criminal syndicate known as the Malavara Entrepreno -- "Bountiful Enterprise" -- who now expect him to act on their behalf.
To ensure that this happens, Malos was assigned a "consultant" named Jakomo Bahun, who's a technical prodigy and hacker, as well as an initiated member of the Malavara Entrepreno. Possessed of an eidetic memory and a devotion to rationality, Jakomo doesn't just keep Malos focused on their mutual employers' business. He also acts as the ship's engineer and keeps its transponder ID changing constantly in order to keep it from being pursued by the Federation Navy. Also aboard ship is Lajla Suleiman, an evangelist for a religion whose exact tenets seem to change every time anyone asks her about them. She claims to have been persecuted for her beliefs on another world of the sector and is thus in need of quick transport elsewhere, but it's pretty clear that Lajla is a con artist possessed of considerable people skills, thereby making her a useful person to have around.
The campaign began with my asking the players to pick a planet on the sector map and I'd take it from there. The chosen world was Nova Nova Kalifornio, which I immediately decided was a rhodium mining world, rhodium being a particularly valuable metal and thus a source of much smuggling to avoid Federation tariffs. The PCs set down on the planet carrying a cargo of "fortified sodium-positive imitation shrimp food" for sale to the miners. In reality, they were there to meet with a criminal contact with whom they could arrange the smuggling of a shipment of rhodium. Naturally, that was far too free of complications to provide a basis for conflict, so I quickly decided that security had been tightened at the mine, thanks to the presence of a new administrator from the megacorp running the place, and the PCs would need to go to the mine and find another way to acquire the shipment in question. thereby kicking the campaign off.
What I discovered is that it's far, far easier to start off an open-ended D&D campaign than an open-ended sci-fi one, especially if the PCs have their own starship. I was lucky in this case because the players decided their characters would be associated with a sector-wide organization. This gave me an easy means to give them hooks into possible adventures, but, unless I choose to be heavy-handed about it -- and I don't -- there's still a great deal of scope for the players to wander very far afield into areas I haven't the slightest notion about. I won't deny that I was more than a little worried about this and so the first session of the campaign, most of which was taken up by character creation, was probably rockier and more tentative than I would have liked, as I struggled to stay one step ahead of the players in creating NPCs and situations for their characters to encounter.
In the end, I think it went very well and am sure future sessions will be more relaxed on my part. Having now laid down some basic information in that first session, I have a better sense of what might exist on Nova Nova Kalifornio and beyond it into the wider sector. I've also got plenty more ideas of hooks to offer the players. As in Dwimmermount, though, I won't be sitting down and filling notebooks with details of worlds and NPCs and adventures before the players make any decisions in play. If anything, I'm even more dedicated to this campaign's being a seat of your pants style affair and, while it'll probably be a bit more harrowing than was Dwimmermount, since I don't even have a dungeon to fall back upon, I also think it has the potential to be a lot more fun for me personally, since I'm getting the chance to referee a sci-fi sandbox, something I've wanted to do for quite some time now.